A reticle is mounted onto a stepper for the transfer of a pattern and used as a mask for a reduced projection exposure apparatus in manufacturing of semiconductor integrated circuits. For example, the reticle has a pattern of a light-shielding film of chromium or the like formed by sputtering, on a transparent glass substrate having at least a main surface mirror-finished.
Nowadays, as miniaturization of patterns progresses, the reticle substrate has been required to have a high flatness and high smoothness.
Recently, the reticle substrate is required to form a pattern with a high accuracy in position of the pattern in an area for the transfer of the pattern. For example, a substrate with 6025 size (6×6×0.25 inches) (1 inch=25.4 mm) is required to have a flatness of 0.5 μm or less (the semiconductor design rule: 100 nm), more preferably, a flatness of 0.25 μm or less (semiconductor design rule: 70 nm) at an area (referred to as a flatness-measuring area, hereinafter) of a main surface of the substrate excluding a peripheral area having a width of 3 mm from an edge or a boundary between the main surface and a chamfered surface. The flatness is defined by a difference in height between the maximum and minimum values of a measuring face, relative to a virtual absolute plane (focal plane) calculated by least-squares method from the measuring face of the substrate surface.
As described above, in the past, requirement of flatness has been directed only to an area except a certain width from the side face of the substrate, i.e. only the central part of the substrate.
However, as miniaturization of patterns progresses, a line width of a pattern has been recently reduced. Therefore, a peripheral configuration of the reticle substrate simply affects the pattern position accuracy when a pattern on the reticle is transferred to a patterning substrate by using a stepper.
Namely, the reticle is generally attached to the stepper so that a main surface having the pattern faces a substrate on which the pattern is transferred. On this occasion, the reticle is fixed so as to secure a broad patterning area and to prevent misalignment of the substrate in the operation of the stepper by vacuum chucking of the periphery outside the flatness-measuring area or the periphery spanning the flatness-measuring area and the area other than the flatness-measuring area of the main surface of the substrate.
FIG. 2 shows a mechanism of attachment of the reticle to the stepper.
With reference to FIG. 2, a reticle 1′ is attached to substrate-holding members 6, and is set to a substrate-holding unit 5. The substrate-holding members 6 are disposed along two edges of the reticle 1′ and are connected to a vacuum unit (not shown). The reticle 1′ is held by suction of the vacuum unit.
On this occasion, if the end shape (flatness, edge flagging, etc.) at the peripheries of the substrate (reticle substrate) constituting a base material of the reticle is improper, the substrate is deformed by vacuum chucking. Therefore, various problems take place in connection with pattern position accuracy in the transferred pattern, i.e. a displacement in distances between the transferred patterns and deterioration of uniformity of line width.
The above-mentioned problems are indicated in Proceedings of SPIE Photomask and Next-Generation Lithography Mask Technology IX, Vol. 4754, 43-53 (2002). Herein, if the edge of the reticle substrate curves upward, the position accuracy decreases when the substrate is held by the stepper The document suggests that the edge of the reticle substrate is preferably flat or has a little edge flagging.
The area of the main surface of the substrate supported by the substrate-holding members 6 is varied at every one of stepper makers. The amount of deformation of the substrate due to the vacuum chucking varies depending on this difference. Therefore, it is necessary that a reticle substrate designed so that the amount of the substrate deformation due to the vacuum chucking is controlled within a predetermined value regardless of the substrate-holding members of the stepper apparatuses. However, it is difficult to actually responding to this. Consequently, the amount of the substrate deformation due to the vacuum chucking must be controlled within a predetermined value, regardless of the areas supported by the substrate-holding members. Such designed reticle substrate is necessary, but the document does not disclose this point.
In general, the reticle substrate is manufactured by precise polishing disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 1-40267.
In the precision polishing, both sides of a plurality of reticle substrates are polished at the same time, the so-called double-side polishing in batch is performed in multistage. The substrates are polished with an abrasive containing cerium oxide as the main ingredient, and then are polished with an abrasive containing colloidal silica as the main ingredient for the finishing. In the precision polishing, a suede-type polishing pad is used for smoothing the main surface of the substrates.
In the above-mentioned method, a high productivity is achieved, however, an excessive pressure is applied to the peripheries of the reticle substrate from the polishing pad. Therefore, the polishing pad sinks and instability in the shape of the polishing pad occurs during the polishing process. Thus, the reticle substrate having a high flatness cannot be obtained. In particular, since the end shape of the reticle substrate becomes improper, the substrate cannot be reliably attached to the substrate-holding means of the stepper. When the pattern on the reticle is transferred to a patterning substrate by using the stepper, the accuracy of pattern position is decreased. As mentioned above, some problems exist.
Consequently, in Japanese Unexamined Patent Application Publication (JP-A) No. 2002-318450, proposal has been made about a method for manufacturing a glass substrate for a photomask. In the method, the glass substrate has a shape so as to have a flat face when the surface of the glass substrate is provided with a patterning of a light-shielding film. So, the exposure of the glass substrate can be performed to the flat face. Such a glass substrate is prepared by partial plasma etching depending on a difference calculated according to shapes of the glass substrate and a glass substrate as a raw material.
In the above-mentioned method, the exposed face of the glass substrate theoretically becomes flat during the exposure. However, since the face roughness and a work-affected layer due to plasma etching occur on the surface of the glass substrate, mechanical polishing must be performed within a very short time. Disadvantageously, a decrease in flatness due to mechanical polishing for the very short time cannot be neglected, and the additional processes reduce the productivity.
The peripheries of the substrate cannot be precisely measured by the method (flatness measurement by optical interference method) for measuring the shape of the substrate disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2002-318450. Therefore, even if a desirable flat face is formed, the shape of the substrate peripheries cannot be actually formed. The reticle is deformed when the reticle is attached to the substrate-holding means of the stepper, and the accuracy in position of the transferred pattern is problematically decreased.
Japanese Unexamined Patent Application Publication (JP-A) No. 2003-51472 discloses a substrate having a flatness of 0.5 μm or less at the peripheral area having a width of 3 mm inward from the edges of the end faces of the substrate. The purposes of this is to prevent a decrease in test sensitivity and to prevent a decrease in accuracy of the face to which resist is applied. Namely, the above-mentioned Japanese Unexamined Patent Application Publication (JP-A) No. 2003-51472 discloses only flatness at the peripheral area of the substrate and does not cite problems when a thin film is formed on the substrate. Japanese Unexamined Patent Application Publication (JP-A) No. 2003-51472 does not refer to the flatness of areas other than the peripheral area of the substrate.
Therefore, if the reticle is configured by forming a thin film on the substrate defined by Japanese Unexamined Patent Application Publication (JP-A) No. 2003-51472, a large film stress is applied to the thin film to deform the shape of the reticle. Furthermore, since the reliability of the flatness value measured at an area near the peripheral area of the substrate is low, deformation of the reticle cannot be sufficiently prevented when the substrate-holding means of steppers are different by the manufacturers.